She was born in Little Lamplight 11 years ago and grew up learning practical medicine from Red, who she considered an older sister. By the time she was 7, she was the preeminent medical authority in Lamplight, and her medical treatment saved Mayor MacCready's life when he was injured in a cave-in. Since then, she's cared a little more about him than the others, and she thinks of Lamplight as "their family." Since MacCready took control, she's been his constant companion, occasionally tempering his instincts with a measure of caution.
At some point, she married MacCready, and had a son with him named Duncan. At an unknown point of time, Lucy is killed by feral ghouls, leaving MacCready and Duncan traveling alone.
Fungus Deal: She reveals information about the curative effects of the mushrooms present in the Little Lamplight settlement, namely, their anti-radiation effects. If Mayor MacCready allows it, she will trade Buffout for cave fungus.
The Replicated Man: If bribed or if the Lone Wanderer can pass a Medicine or Speech check, she will say that Pinkerton is the one who pulled off the android's surgery (it is strange she knows this, as she is a child and no one has seen Pinkerton in years, as well as the fact that the Little Lamplighters have limited contact with the outside world).
In Lucy's clinic in the office building, there is a banner reading, "The doctor is in!" This is a reference to the character Lucy from the Peanuts comic strip (who pretends to be a psychiatric doctor).